This invention relates to reversible hydrogenation reactions usable for H.sub.2 separation or storage or in chemical synthesis, and particularly to a class of bridged bimetallic transition metal compounds that are useful as catalysts for such reactions.
The chemical reactions of molecular hydrogen have use in the hydrogenation of unsaturated hydrocarbons and similar compounds, and in processes for H.sub.2 separation and storage. In particular, the selective separation or purification of H.sub.2 from the product gas of coal gasification would be highly desirable. However, gas separation has in the past been a difficult and energy intensive process. The development of new and innovative techniques for selectively and efficiently separating specific gas components from mixed-gas streams would significantly reduce the cost and complexity of product gas production and processing. For example, efficient H.sub.2 separation from synthesis gas could help to make coal an attractive future source of H.sub.2 for use as a fuel or chemical feedstock. In addition, this technology could have a significant impact on processes not directly associated with coal gasification in which H.sub.2 is lost in waste streams. These processes may include ammonia manufacture, reduction of metallic oxide ores, and the hydrogenation of fats and oils. Thus, wide-ranging applications exist for H.sub.2 separation and recovery technologies.
However, the chemical reactions used either for separation or purification of H.sub.2 must be highly reversible and selective in order to be commercially attractive. The reactions that are currently used for such purposes tend to be either inefficient or non-selective. For example, the recovery of H.sub.2 from Pressure Swing Adsorption (PSA) is on the order of only 80%. Furthermore, PSA is ineffective with feeds containing less than 50% of H.sub.2. Membrane separation systems are inherently energy efficient, but existing commercial systems such as PRISM cannot separate H.sub.2 from streams containing gases such as CO.sub.2.
A significant factor in determining the efficiency and yield of any process for storing or recovering hydrogen is the type of catalyst employed. For example, U.S. Pat. No. 4,695,446 issued Sept. 22, 1987 to Bogdanovic, describes a method of separating H.sub.2 from a hydrogen-poor gas mixtures by contacting the mixture with a type of "active" magnesium which has been doped with a transition metal. The active magnesium reacts with hydrogen from the mixture to form magnesium hydride. Hydrogen may then be recovered by thermally dehydrogenating the magnesium hydride. However, it is preferred to use homogeneous catalytic processes in order to provide commercially acceptable rates of reaction.
The bulk of the research with respect to hydrogenation catalysts has related to chemical synthesis and especially the hydrogenation of unsaturated compounds. A number of such hydrogenation catalysts have included transition metal organometallic complexes.
For example, U.S. Pat. No. 4,645,849 issued Feb. 24, 1987 to Lewis describes a method for hydrogenating olefins and alkynes wherein the unsaturated hydrocarbon is reacted with H.sub.2 in the presence of certain "cyclometallated" complexes in which a transition metal such as palladium, platinum, cobalt or the like forms a 4-6 membered ring system with a covalently bonded carbon in a hydrocarbon chain and a ligand atom such as phosphorous, nitrogen, arsenic, oxygen or sulfur. Additionally, U.S. Pat. No. 4,670,621 issued June 2, 1987 to Walker describes the use of certain cyclopentadienyl transition metal complexes of iridium and osmium as dehydrogenation catalysts in the conversion of paraffins to olefins having a corresponding carbon skeleton. Also, U.S. Pat. No. 4,360,475 issued Nov. 23, 1982 to Pruett, et al describes a class of bimetallic cluster compounds consisting of ruthenium carbido octahedra linked through a Group IIIA metal atom which may be used for converting synthesis gas to hydrocarbons and oxygenates.
Several organometallic catalysts which have been used for purposes other than hydrogenation (or dehydrogenation) have employed the cyclopentadienyl group as a ligand. For example, U.S. Pat. No. 4,423,276 issued Dec. 27, 1983 to Johnson describes a class of cyclopentadienyl tantalum compounds useful for olefin isomerization, i.e., the switching of an internal double bond to a terminal double bond. The process seems to be peculiar to the Ta atom. Similarly, U.S. Pat. No. 4,153,576 issued May 8, 1979 to Karol, et al relates to the preparation of cyclopentadienyl chromium alkyl/aryl oxides and siloxides, for use primarily as catalysts in the polymerization of ethylene but also for use in scavenging oxygen and volatile sulfur compounds from various liquid and gaseous streams. However, these compounds must be used on a silicon support and activated with a silane. U.S. Pat. No. 4,086,408 issued Apr. 25, 1978 to Karol, et al further describes the modification of bis-cyclopentadienyl chromium [II] compounds used as supported catalysts on an inorganic oxide. These complexes are modified by addition thereto of oxygen containing compounds such as ethers in order to improve the impact strength and toughness of ethylene polymers made therewith. Also, Japanese Patent No. J6 0092-297-A describes a catalyst for the synthesis of pyridine derivatives from alkynes and nitriles which may be produced by the reaction of eta 5-substituted cyclopentadienyl cobalt monohalides with polyenes having 1-16 carbon atoms.
In reactions with molecular hydrogen, the use of bimetallic complexes as catalysts has sometimes been preferred. For example, U.S. Pat. No. 4,605,751 issued Aug. 12, 1986 to Curtis, et al describes a class of heterobimetallic cluster compositions usable for the selective hydrogenation of carbon monoxide. The complex includes sulfur directly bonded into the cluster, a metal from the group consisting of Cr, Co and W, another metal from the group Fe, Co and Ni and two or more cyclopentadienyl groups. The catalyst is supported on a refractory base such as alumina. Also, U.S. Pat. No. 4,656,299 issued Apr. 7, 1987 to Fujii, et al describes a class of substituted cyclopentadienyl cobalt complexes including certain bimetallics that are usable as catalysts in the preparation of pyridine homologues. The bimetallic form of this complex has two cobalt atoms bonded directly together and further connected through a metallocyclic ring.
U.S. Pat. No. 4,361,497 issued Nov. 30, 1982 to Boldt, et al represents one instance in which catalysts of the cyclopentadienyl metal carbonyl type have been used for hydrogenation. However, these have been polymeric catalysts in which the metal complex is bound to a polymer carrier, either directly through the cyclopentadiene group or indirectly through a methylene bridge, a phenyl group or the like in lieu of a phosphine type linkage. However, as noted earlier, the use of homogeneous catalysts is preferred.
Homogeneous hydrogenation catalysts based upon bimetallic complexes of rhodium or iridium in combination with methyl-substituted cyclopentadienyl groups are described in U.S. Pat. No. 3,849,459 issued Nov. 19, 1974 to Maitlis, et al. A catalytic hydride is postulated to have a hydrogen atom in an unusual bridging position between the two metal atoms with further bridging between the metals being provided by Cl atoms. The catalytic activity described in the patent is attributed to the bridged hydrogen.
Other classes of bimetallic compounds have been described for which no catalytic activity has been indicated. For example, U.S. Pat. No. 3,097,153 issued July 9, 1963 to Hubel, et al discloses a class of organometallic complexes which include metals of the VI, VII, or VIII subgroups of the periodic table, including Cr. This class may include bimetallic species associated with multiple carbonyl groups. The various complexes are used to produce hydrogenated linear and cyclic organic compounds that are free of carbon-to-metal bonds by reaction with hydrogenation agents such as LiAlH.sub.4. Similarly, U.S. Pat. No. 3,326,948 issued June 20, 1967 to Cais, et al describes a variety of ferrocene-like organometallic compounds which include cyclopentadienyl groups that are .pi.-bonded to transition metal carbonyl groups in which the metal may be Cr, Fe or Mn. The class also includes a number of bimetallic compounds. However, Cais et al require that olefinic chain components be connected to the cyclopentadienyl groups.
The art as referenced above thus does not provide any more efficient or rapid means of removing H.sub.2 from mixed gas streams or for reversibly storing hydrogen. Furthermore, it would be useful to identify a class of compounds that would not only store hydrogen, but would also serve as hydrogenation catalysts through such a hydrogen-storing mechanism. The present invention provides such a class of compounds.